Abstract:

Ultrasonic Guided Waves can propagate over long distances, and are thus suitable
for the interrogation of long structural members such as rails. A recently developed
Ultrasonic Broken Rail Detection (UBRD) system for monitoring continuously welded
train rail tracks, primarily detects complete breaks. This system uses a guided wave
mode with energy concentrated in the head of the rail, which propagates large distances
and which is suitable for detecting defects in the rail head. Exploiting a second mode,
with energy concentrated in the web section, would allow us to e ectively detect defects
in the web of the rail.
The objective of this study is to develop an ultrasonic piezoelectric transducer that can
excite a guided wave mode with energy concentrated in the web of the rail. It is required that the transducer must strongly excite such a mode at the operational frequency of the
UBRD system. The objective is thus to obtain a design with optimal performance.
A recently developed numerical modelling technique is used to model the interaction
of the transducer with the rail structure. The technique employs a 2D Semi-Analytical
Finite Element (SAFE) mesh of the rail cross-section and a 3D nite element mesh of the
transducer; and is thus referred to as SAFE-3D. The accuracy of the SAFE-3D method
was validated though experimental measurements performed on a previously developed
transducer.
A design objective function representative of the energy transmitted by the transducer
to the web mode was selected. The identi ed design variables were the dimensions of
the transducer components. The performance of the transducer was optimized using a
response surface-based optimization approach with a Latin Hypercube sampled design of
experiments (DoE) that required SAFE-3D analyses at the sampled points. A Nelder-
Mead optimization algorithm was then used to nd an optimal transducer design on the
response surface.
The performance of the optimal transducer predicted by the response surface was
found to be in good agreement with that computed from SAFE-3D. The optimum transducer
was manufactured and experimental measurements veri ed that the transducer
model was exceptionally good. The design method adopted in this study could be used
to automate the design of transducers for other sections of the rail or other frequencies
of operation.